Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

Paul E. McKenney
2009-04-17T14:51:49Z

On Fri, Apr 17, 2009 at 01:44:51AM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Thu, Apr 16, 2009 at 10:19:02PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > On Thu, Apr 16, 2009 at 04:49:55PM -0700, Paul E. McKenney wrote:

[ . . . ]

> > > > +
> > > > +void synchronize_rcu_fgp(void)
> > > > +{
> > > > + mutex_lock(&rcu_fgp_mutex);
> > > > +
> > > > + /* CPUs must see earlier change before parity flip. */
> > > > + smp_call_function(rcu_fgp_do_mb, NULL, 1);
> > >
> > > /*
> > > * Call a function on all other processors
> > > */
> > > int smp_call_function(void(*func)(void *info), void *info, int wait);
> > >
> > > I guess you meant on_each_cpu ? That should include "self", given we
> > > also need the smp_mb().
> >
> > Hmmm... Why do we need "self"? Because synchronize_rcu_fgp() blocks,
> > it had jolly well better not be within a read-side critical section.
> >
> > So, what am I missing here?
>
> I mean that I think we also need some smp_mb()s on the writer side,
> don't we ? If we want the changes done by the writer (assign pointer) to
> be shown to the readers before the writer starts flipping the parity, a
> smp_mb() is needed at the beginning of synchronize_rcu_fgp() (actually
> at the same location where you call the rcu_fgp_do_mb ipis), same at the
> end (so we order parity flipping with the next assign pointer).
>
> Or maybe it's getting late and I am missing the obvious.

The smp_call_function() itself must have barriers in order to ensure
that the other CPUs see the updates to its parameter block.

But see my upcoming response to Dave and Peter.

Thanx, Paul

> Mathieu
>
> > > > +
> > > > + /*
> > > > + * We must flip twice to correctly handle tasks that stall
> > > > + * in rcu_read_lock_fgp() between the time that they fetch
> > > > + * rcu_fgp_ctr and the time that the store to their CPU's
> > > > + * rcu_fgp_active_readers. No matter when they resume
> > > > + * execution, we will wait for them to get to the corresponding
> > > > + * rcu_read_unlock_fgp().
> > > > + */
> > > > + ACCESS_ONCE(rcu_fgp_ctr) ^= RCU_FGP_PARITY; /* flip parity 0 -> 1 */
> > > > + rcu_fgp_wait_for_quiescent_state(); /* wait for old readers */
> > > > + ACCESS_ONCE(rcu_fgp_ctr) ^= RCU_FGP_PARITY; /* flip parity 1 -> 0 */
> > > > + rcu_fgp_wait_for_quiescent_state(); /* wait for old readers */
> > > > +
> > > > + /* Prevent CPUs from reordering out of prior RCU critical sections. */
> > > > + smp_call_function(rcu_fgp_do_mb, NULL, 1);
> > > > +
> > >
> > > Same as above.
> >
> > Same as above. ;-)
> >
> > > Mathieu, who can still recognise his original userspace implementation
> > > :-)
> >
> > Yeah, I never was all that good at disguising code anyway. But I did
> > keep a couple of changes. ;-)
> >
> > Updated patch below.
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > And here is a crude second cut. Untested, probably does not even compile.
> >
> > Straight conversion of Mathieu Desnoyers's user-space RCU implementation
> > at git://lttng.org/userspace-rcu.git to the kernel (and yes, I did help
> > a little, but he must bear the bulk of the guilt). Pick on srcu.h
> > and srcu.c out of sheer laziness. User-space testing gives deep
> > sub-microsecond grace-period latencies, so should be fast enough, at
> > least if you don't mind two smp_call_function() invocations per grace
> > period and spinning on each instance of a per-CPU variable.
> >
> > Again, I believe per-CPU locking should work fine for the netfilter
> > counters, but I guess "friends don't let friends use hashed locks".
> > (I would not know for sure, never having used them myself, except of
> > course to protect hash tables.)
> >
> > Most definitely -not- for inclusion at this point. Next step is to hack
> > up the relevant rcutorture code and watch it explode on contact. ;-)
> >
> > Changes since v1:
> >
> > o Applied Mathieu's feedback.
> >
> > o Added docbook headers and other comments.
> >
> > o Added the rcu_fgp_batches_completed API required by rcutorture.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >
> > include/linux/srcu.h | 42 ++++++++++++++++++++++++
> > kernel/srcu.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > 2 files changed, 131 insertions(+)
> >
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68